1. Field of the Invention
This invention relates to a warning indicator that senses tire pressure and emits an audible alarm when a low tire air pressure is sensed.
2. Description of the Prior Art
U.S. Pat. Nos. 1,455,146; 3,929,090; and 4,024,829 disclose tire pressure warning devices that sense tire air pressure and, upon sensing a low tire pressure condition, blow up an elastomeric member that pops to provide an audible sound indicating such condition. In the first two listed patents above and all but one embodiment of the last patent, the devices each include a valve mechanism that works against the normal tire valve through which air is supplied and exhausted into and out of the tire. When tire pressures are within the normal range, the tire valve has its valve element biased outwardly with a sufficient force to resist a bias of the valve mechanism tending to move the tire valve element inwardly to open the tire valve. However, when low tire pressure conditions are present, the tire pressure valve element bias in the outward direction is not great enough to counteract the inward bias of the valve mechanism and the tire valve is then opened to allow pressurized tire air to flow outwardly and blow up the elastomeric member in order to cause it to rupture and pop. With this type of warning device, it is difficult to achieve uniform pressure sensing due to manufacturing variances of the tire valves with which the device is used. Different tire valves of the same design and construction may require different forces to open their valve elements and thereby permit the air flow that blows up the elastomeric member which ruptures to provide the audible alarm indicating a low tire pressure condition.
One embodiment of the U.S. Pat. No. 4,024,829 includes a valve opener that opens the tire valve element as the warning device is secured to the tire valve. Pressurized tire air which flows to the warning device must pass a pressure sensing valve before passage to a chamber enclosed by the elastomeric member which ruptures. A spring biased valve actuator biases a valve element of the pressure sensing valve and acts against the pressurized air from the tire to sense its pressure and unseat the valve element when a low pressure condition is sensed. It has been found that problems can occur if this warning device is not made with a two-piece construction since the spring biased valve actuator can maintain the valve element of the pressure sensing valve open as the warning device is attached to the tire valve. A connector subassembly can be initially secured to the tire valve prior to securing the indicator to the subassembly. However, as the warning device is secured to the connector subassembly, the spring biased actuator can engage the valve element of the pressure sensing valve and move it so that air flows past the pressure sensing valve to blow up the elastomeric member even though the tire air pressure is not low. If a threaded connection is used to secure the warning device to the connector subassembly, friction generated between the spring biased valve actuator of the warning device can tend to rotate the pressure sensing valve element and thereby cause leakage which likewise blows up the elastomeric member.
Other tire pressure sensing devices are disclosed by U.S. Pat. Nos. 1,236,411; 1,450,769; 2,037,969; 2,795,204; 3,276,417; 3,111,930; 3,489,167; 3,536,026; 3,572,283; 3,680,523; 3,738,308; 3,776,174; and 3,827,393.